Chromium-free paint composition and paint films obtained by coating same

ABSTRACT

To provide a chromium-free paint composition which can provide paint films which are excellent in terms of corrosion resistance on the edge surfaces, scratched surfaces and worked parts of a pre-coated steel sheet, and the paint films which can be obtained by coating this chromium-free paint composition. A chromium-free paint composition which includes paint film-forming resin (A), anti-rust pigment (B), comprising at least one type of amorphous MgO—V 2 O 5 -based compound, and crosslinking agent (C) which is characterized in that the mass content proportion of the aforementioned anti-rust pigment (B) is from 10 to 80 mass % with respect to the sum of the resin solid fraction mass of the aforementioned paint film-forming resin (A) and the aforementioned crosslinking agent (C), and the overall eluted ion content in a 10% aqueous solution of the aforementioned anti-rust pigment (B) is from 10 ppm to 100 ppm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage entry of PCT/IB2012/002623, filedon Dec. 4, 2012, which claims priority to Japanese Application Number2011-282396, filed on Dec. 22, 2011, which are incorporated herein byreference in their entireties.

TECHNICAL FIELD

The invention concerns chromium-free paint compositions which aresuitable for use as undercoat paints in the production of pre-coatedsteel sheets.

BACKGROUND TECHNOLOGY

The pre-coated steel sheet is generally produced with a two-coattwo-bake system in which an undercoat paint is coated on a base steelsheet as typified by a zinc-based plated steel sheet after carrying outa chemical forming treatment as a pre-treatment, heated and cured andthen a top-coat paint is coated, heated and cured. Pre-coated steelsheets which have been produced in this way are delivered initially tothe user in the form of a coil. Hence the user unwinds the coil and cutsoff the amount which is required for the application and works this intovarious shapes for use.

The pre-coated steel sheet is cut and subjected to working after thepaint film has been formed and so there are end surfaces and oftencracks and scratches in the worked parts where the metal is exposedlocally and a lowering of corrosion resistance and peeling of the paintfilm are liable to arise in such parts. Hence, in the past the basesteel sheet has been subjected to a chemical forming treatment whichincludes chromate and, moreover, an undercoat paint which contains achromium-based anti-rust pigment has been applied in order to ensurecorrosion resistance and adhesion of the paint film on the base steelsheet.

However, in recent years the adverse effects on the environment due tothe dissolving out of highly toxic chromium have been regarded as aproblem and there is a demand for chromium-free paint compositions inwhich no chromium-based anti-rust pigment is used as undercoat paints.

Paints in which vanadium compounds are used as anti-rust pigments areknown as chromium-free paint compositions (for example Patent Documents1 to 4).

A chromium-free paint composition which is characterized by containingsilica particles which have a specified average particle diameter, oiluptake and pore volume and magnesium salts which include magnesiumvanadate has been disclosed in Patent Document 1. Furthermore, achromium-free paint composition in which metal silicate and a metalhydrogen phosphate are used conjointly with at least one type ofvanadium compound selected from among vanadium pentoxide, calciumvanadate and magnesium vanadate as anti-rust pigments has been disclosedin Patent Document 2.

Furthermore, a chromium-free paint composition in which molybdenumoxide, metal silicate, phosphoric acid-based metal salt and at least onetype of vanadium compound from among vanadium pentoxide, calciumvanadate and magnesium vanadate are used conjointly as anti-rustpigments has been disclosed in Patent Document 3.

However, the chromium-free paint compositions described in PatentDocuments 1 to 3 provide poor corrosion resistance when compared withpaints in which chromium-based pigments are used and the corrosionresistance on the end surfaces in particular is inadequate. Furthermore,the water resistance is often poor when anti-rust pigments are used inlarge amounts and so this has not resulted in the substitution ofchromium-based anti-rust pigments in the production of pre-coated metalsheets. Furthermore, the processes are complicated since a plurality ofanti-rust pigments must be added.

Moreover, it is disclosed in Patent Document 4 that paint films withwhich corrosion resistance and moisture resistance are improved can beformed by setting the conductivity and pH of a 1 mass % aqueous solutionof calcium vanadate in an anti-rust paint composition which containscalcium vanadate as an anti-rust pigment within specified ranges.

However, the paint compositions described in Patent Document 4 providepoor long-term corrosion resistance in corrosive environments which havea high wet rate such as salt-water spray tests when compared with paintsin which chromium-based pigments are used, and the corrosion resistancein worked parts and at the end surface parts in particular isunsatisfactory. Moreover, when used for a coated steel sheet where thebase material comprises 55% Al—Zn-based melt plated steel sheet, whiterust and red rust formation is formed from the end surface part even inthe early stages of corrosion depending on the pH of the calciumvanadate and satisfactory corrosion resistance is not obtained and sothis has not resulted in the substitution of chromium-based anti-rustpigments in the production of pre-coated metal sheets.

PRIOR ART LITERATURE Patent Documents

-   Patent Document 1:-   Japanese unexamined patent application laid open 2001-172570-   Patent Document 2:-   Japanese unexamined patent application laid open 2008-291160-   Patent Document 3:-   Japanese unexamined patent application laid open 2008-291162-   Patent Document 4:-   Japanese unexamined patent application laid open 2011-184624

OUTLINE OF THE INVENTION Problems to be Resolved by the Invention

Hence, the present invention is intended to provide a chromium-freepaint composition which provides paint films which are excellent interms of corrosion resistance on the end surfaces, worked parts andscratched parts of pre-coated steel sheets.

Means of Resolving these Problems

As a result of thorough research carried out with a view to resolvingthe problems outlined above the inventors have discovered that theabovementioned problems can be resolved with a chromium-free paintcomposition which includes anti-rust pigment comprising at least onetype of amorphous MgO—V₂O₅-based compound. That is to say, the presentinvention provides a chromium-free paint composition which includespaint film-forming resin (A), anti-rust pigment (B) comprising at leastone type of amorphous MgO—V₂O₅-based compound and crosslinking agent (C)which is characterized in that the mass content proportion of theaforementioned anti-rust pigment (B) is from 10 to 80 mass % withrespect to the sum of the resin solid fraction mass of theaforementioned paint film-forming resin (A) and the aforementionedcrosslinking agent (C) and that the total eluted ion content of a 10%aqueous solution of the aforementioned anti-rust pigment (B) is from 10ppm to 100 ppm.

Furthermore, the present invention provides a chromium-free paintcomposition in which, in the abovementioned chromium-free paintcomposition, the pH of a 10 mass % aqueous solution of theaforementioned anti-rust pigment (B) is from 9.0 to 11.0.

Furthermore, the present invention provides a chromium-free paintcomposition in which, in the abovementioned chromium-free paintcomposition, the aforementioned anti-rust pigment (B) includes vanadiumand magnesium as structural elements and the mol ratio of theaforementioned magnesium with respect to the aforementioned vanadium isfrom 1.7 to 5.0.

Furthermore, the present invention provides a chromium-free paintcomposition in which, in the abovementioned chromium-free paintcomposition, the magnesium compound which is a raw material of theaforementioned amorphous MgO—V₂O₅-based compound is at least one type ofmagnesium-containing compound selected from magnesium oxide andmagnesium carbonate.

Furthermore, the present invention provides a chromium-free paintcomposition in which, in the abovementioned chromium-free paintcomposition, the vanadium-containing compound which is a raw material ofthe aforementioned amorphous MgO—V₂O₅-based compound is vanadiumpentoxide.

Furthermore, the present invention provides a chromium-free paintcomposition in which, in the abovementioned chromium-free paintcomposition, the aforementioned paint film-forming resin (A) is at leastone type of paint film-forming resin selected from the hydroxylgroup-containing epoxy resins of number average molecular weight from400 to 10,000 and hydroxyl-group containing polyester resins of numberaverage molecular weight from 500 to 20,000.

Furthermore, the present invention provides a chromium-free paintcomposition in which, in the abovementioned chromium-free paintcomposition, the aforementioned crosslinking agent (C) is at least onetype of crosslinking agent selected from the blocked polyisocyanatecompounds and amino resins and the solid fraction mass contentproportion of the aforementioned crosslinking agent (C) is from 3 to 60mass % with respect to the solid fraction mass of the aforementionedpaint film-forming resin (A).

Furthermore, the present invention provides paint films obtained bycoating the abovementioned chromium-free paint compositions.

Effect of the Invention

It is possible with a chromium-free paint composition of this inventionto obtain paint films which are excellent for corrosion resistance inthe end surface, scratched surface and worked parts of a pre-coatedsteel sheet.

Embodiment of the Invention

The paint film-forming resin (A) which is used in a chromium-free paintcomposition of this invention is not subject to any particularlimitation save that it is a resin which has a paint film-formingcapacity and which has functional groups which can react with thecrosslinking agent (C) but, from the viewpoints of workability andadhesion on the base steel sheet, it is preferably at least one type ofpaint film-forming resin selected from the epoxy resins and polyesterresins. These paint film-forming resins can be used individually andcombinations of two or more types can also be used.

In those cases where an epoxy resin is used for the paint film-formingresin (A) there are a bisphenol A-type epoxy resins synthesized frombisphenol A and epichlorhydrin and bisphenol F-type epoxy resinssynthesized from bisphenol F and epichlorhydrin as epoxy resins but,from the viewpoint of corrosion resistance, the bisphenol A-type epoxyresins are preferred.

In those cases where an epoxy resin is used for the paint film-formingresin (A) the number average molecular weight of the epoxy resin is,from the viewpoints of workability, corrosion resistance and coatingoperability, preferably from 400 to 10,000, more desirably from 400 to9,000 and most desirably from 400 to 8,000. Moreover, the value of thenumber average molecular weight in this invention is that obtained bymeans of gel permeation chromatography (GPC) with polystyrene as thestandard substance. Furthermore, no particular limitation is imposedupon the epoxy equivalent of the epoxy resin in those cases where anepoxy resin is used for the paint film-forming resin (A) but it is, forexample, preferably from 180 to 5,000.

In those cases where an epoxy resin is used for the paint film-formingresin (A) all or some of the epoxy groups of this epoxy resin may havebeen modified by reaction with a modifying agent. Examples of the epoxyresin modifying agents include polyester, alkanolamine, caprolactone,isocyanate compound, phosphoric acid compound, acid anhydride and thelike. These modifying agents can be used individually and combinationsof two or more types can also be used.

In those cases where a polyester resin is used for the paintfilm-forming resin (A) the polyester resin can be obtained by means of aknown method using the reaction of polyhydric alcohols and polybasicacids.

Glycols and polyhydric alcohols which have three or more hydroxyl groupscan be cited as polyhydric alcohols. Examples of the glycols includeethylene glycol, propylene glycol, diethylene glycol, triethyleneglycol, tetraethylene glycol, dipropylene glycol, polyethylene glycol,polypropylene glycol, neopentyl glycol, hexylene glycol, 1,3-butanediol,1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol,2-butyl-2-ethyl-1,3-propanediol, methyipropanediol,cyclohexanedimethanol, 3,3-ditheyl-1,5-pentanediol and the like.Furthermore, examples of the polyhydric alcohols which have three ormore hydroxyl groups include glycerol, trimethylolethane,trimethylol-propane, pentaerythritol, dipentaerythritol and the like.These polyhydric alcohols can be used individually and combinations oftwo or more types can also be used.

Generally polycarboxylic acids are used for the polybasic acid butmonocarboxylic acids and the like can be used conjointly, as required.Examples of the polycarboxylic acids include phthalic acid,tetrahydrophthalic acid, hexahydrophthalic acid,4-methylhexahydrophthalic acid, bicyclo[2,2,1]heptane-2,3-dicarboxylicacid, trimellitic acid, adipic acid, sebacic acid, succinic acid,azeleic acid, fumaric acid, maleic acid, itaconic acid, pyromelliticacid, dimer acid and the like, the anhydrides of these acids, and1,4-cyclohexanedicarboxylic acid, isophthalic acid,tetrahydroisophthalic acid, hexahydroisophthalic acid,hexahydroterephthalic acid and the like. These polybasic acids can beused individually and combinations of two or more types can also beused.

In those cases where a polyester resin is used for the paintfilm-forming resin (A) the hydroxyl group value of the polyester resinis, from the viewpoints of solvent resistance, workability and the like,preferably from 5 to 200 mgKOH/g, more desirably from 7 to 150 mgKOH/gand most desirably from 10 to 130 mgKOH/g.

In those cases where a polyester resin is used for the paintfilm-forming resin (A) the number average molecular weight of thepolyester resin is, from the viewpoints of solvent resistance,workability and the like, preferably from 500 to 20,000, more desirablyfrom 700 to 18,000 and most desirably from 800 to 16,000.

Furthermore, no particular limitation is imposed upon the acid value ofthe polyester resin in those cases where a polyester resin is used forthe paint film-forming resin (A), but it is, for example, preferablyfrom 0 to 10 mgKOH/g.

A chromium-free paint composition of this invention contains ananti-rust pigment (B) comprising at least one type of amorphousMgO—V₂O₅-based compound.

The amorphous MgO—V₂O₅-based compounds which can be used in theinvention can be obtained by means of a know method of production andthe method of production with which they are obtained by mixing andreacting magnesium-containing compounds and vanadium-containingcompounds in water can be cited as an example. In this case the reactionproduct is subjected to treatments such as washing with water,de-watering, drying, pulverization and the like.

Magnesium oxide and various magnesium salts can be used for themagnesium-containing compounds which form the raw materials forproducing amorphous MgO—V₂O₅-based compounds which can be used in thisinvention. In more practical terms examples include magnesium oxide,magnesium carbonate, magnesium hydroxide, magnesium chloride, magnesiumnitrate, magnesium acetate, magnesium sulfate and the like. From amongthese magnesium oxide and magnesium carbonate in particular arepreferred. These magnesium-containing compounds can be used individuallyand combinations of two or more types can also be used.

Vanadium oxide and various vanadium salts can be used for thevanadium-containing compounds which form the raw materials for producingamorphous MgO—V₂O₅-based compounds which can be used in this invention.In more practical terms examples include vanadium pentoxide, potassiumvanadate, sodium vanadate, ammonium vanadate and the like. From amongthese vanadium pentoxide in particular is preferred. Thesevanadium-containing compounds can be used individually and combinationsof two or more types can also be used.

By selecting appropriate raw materials from among thesemagnesium-containing compounds and vanadium-containing compounds it ispossible to obtain the desired amorphous MgO—V₂O₅-based compounds. Atleast one type of amorphous MgO—V₂O₅-based compound which has beenobtained in this way is used as the anti-rust pigment (B) in achromium-free paint composition of this invention.

The anti-rust pigment (B) of this invention includes vanadium andmagnesium as structural elements and the mol ratio of magnesium withrespect to vanadium is, from the viewpoint of corrosion resistance,preferably rom 1.7 to 5.0, more desirably from 2.0 to 5.0 and mostdesirably from 2.3 to 4.5.

Moreover, in those cases where two or more types of amorphousMgO—V₂O₅-based compounds are used for the anti-rust pigment (B) the molratio of magnesium with respect to vanadium in the anti-rust pigment (B)is determined on the basis of the total amounts of vanadium andmagnesium included as structural elements in each of the amorphousMgO—V₂O₅-based compounds.

In terms of the anti-rust pigment (B) of this invention, it is possibleto obtain a paint film with which high corrosion resistance can bemaintained for a prolonged period of time by having the total eluted ioncontent of a 10 mass % aqueous solution of the anti-rust pigment (B)within a specified range.

A 10 mass % aqueous solution of the anti-rust pigment (B) is prepared byadding 10 g of the anti-rust pigment (B) in 90 g of ion-exchanged waterand preparing a liquid suspension and then vigorously shaking the liquidsuspension for 1 minute and leaving it to stand at room temperature for24 hours.

The total eluted ion content of the 10 mass % aqueous solution of theanti-rust pigment (B) in this invention is the value obtained byextracting the supernatant liquid of the 10 mass % aqueous solution ofthe anti-rust pigment (B) prepared in the way outlined above andmeasurement using ICP emission analysis apparatus (JY-238 Ultrace,produced by the Horiba Seisakujo Co.).

The total eluted ion content of the 10 mass % aqueous solution of theanti-rust pigment (B) in this invention is from 10 to 100 ppm and, fromthe viewpoint of being able to maintain high corrosion resistance over along period of time, it is preferably from 15 to 90 ppm and moredesirably from 20 to 80 ppm.

If the total eluted ion content is less than 10 ppm then there are caseswhere the corrosion resistance declines, and if it exceeds 100 ppm thena lowering of water resistance arises and there are cases where thecorrosion resistance declines.

No particular limitation is imposed upon the grain size of the anti-rustpigment (B) comprising amorphous MgO—V₂O₅-based compounds of thisinvention but it is suitably within the range from 10 to 30 μm.Moreover, the grain size is obtained as the particle diameter showing a50% frequency (mid-grain size) in the grain size distribution measuredwith laser diffraction/scattering-type grain size distribution measuringapparatus (trade name LA-920, produced by the Horiba Seisakujo Co.).

The anti-rust pigment (B) of this invention exhibits optimum elutionproperties and solubility and so high corrosion resistance can bemaintained over a long period of time even without the conjoint use ofother anti-rust pigments and, moreover, corrosion not just of the coatedsurface of the object which has been painted but also of the end surfaceparts can be prevented effectively.

It is possible to obtain paint films which have even better corrosionresistance by having the pH of a 10 mass % aqueous solution of theanti-rust pigment (B) of this invention within a specified range.

The pH of a 10 mass % aqueous solution of an anti-rust pigment (B) inthis invention is the value measured with a pH meter (HM-20S, producedby the Toa Denpa Kogyo Co.) on extracting the supernatant liquid of a 10mass % aqueous solution of the anti-rust pigment (B) prepared with themethod described above.

From the viewpoint of corrosion resistance the pH of a 10 mass % aqueoussolution of an anti-rust pigment (B) of this invention is preferablyfrom 9.0 to 11.0, more desirably from 9.5 to 11.0 and most desirablyfrom 10.0 to 11.0. By using an anti-rust pigment (B) of which the pH ofa 10 mass % aqueous solution is within the ideal range it is possible toobtain paint films which exhibit high corrosion resistance even in thosecases where the object being painted is a plated steel sheet whichincludes zinc or aluminum since, as a result of the pH buffering action,a pH value in the region in which the anodic reaction of zinc occurs (pHfrom 6 to 8) is unlikely to arise.

The mass content proportion of the anti-rust pigment (B) is from 10 to80 mass % with respect to the sum of the resin solid fraction masses ofthe aforementioned paint film-forming resin (A) and the aforementionedcrosslinking agent (C) and, from the viewpoint of the corrosionresistance, it is more desirably from 15 to 80 mass % and most desirablyfrom 20 to 80 mass %.

If the mass content proportion of the anti-rust pigment (B) is less than10 mass % then there are cases where the corrosion resistance isunsatisfactory, and if it exceeds 80 mass % then there are cases wherethe mechanical properties and adhesion on base material steel sheetdecline.

With the inclusion of an anti-rust pigment (B) comprising at least onetype of amorphous MgO—V₂O₅-based compound, a chromium-free paintcomposition of this invention exhibits excellent corrosion resistancewith just the anti-rust pigment (B) without the conjoint use of otheranti-rust pigments, but other chromium-free type anti-rust pigments maybe used conjointly, as required. Examples of other chromium-free typeanti-rust pigments include molybdate pigments, phosphomolybdatepigments, calcium-silica-based pigments, phosphate-based pigments,silicate-based pigments and the like.

The crosslinking agents (C) which are used in this invention react withthe paint film-forming resin (A) to form a cured paint film. Aminoresins, polyisocyanate compounds, blocked polyisocyanate compounds andthe like can be cited as examples of the crosslinking agent (C), butfrom the viewpoints of workability and generality the melamine resinsand blocked polyisocyanate compounds are preferred. These crosslinkingagents can be used individually and combinations of two or more typescan also be used.

Amino resin is a general term signifying resins where formaldehyde hasbeen added to compounds which include amino groups and condensed, and inmore practical terms examples of these include melamine resins, urearesins, guanamine resins and the like. From among these the melamineresins are preferred. The partially or fully methylolated melamineresins obtained by reacting melamine and formaldehyde, the alkyletherified type melamine resins obtained by partially or completelyetherifying some or all of the methylol groups of the methylolatedmelamine resins with an alcohol component, imino group-containingmelamine resins and mixtures of these types of melamine resin can becited as examples of the melamine resins. Examples of the alkyl ethertype melamine resins include methylated melamine resins, butylatedmelamine resins, methyl/butyl mixed alkyl type melamine resins and thelike.

Examples of the polyisocyanate compounds include aliphatic diisocyanatessuch as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate,dimer acid diisocyanate and the like, and also cyclic aliphaticdiisocyanates such as isophorone diisocyanate, xylylene diisocyanate(XDI), m-xylylene diisocyanate, hydrogenated XDI and the like and,moreover, aromatic diisocyanates such as tolylene diisocyanate (TDI),4,4-diphenylmethane diisocyanate (MDI), hydrogenated TDI, hydrogenatedMDI and the like, and adducts, biuret forms and isocyanurate forms ofthese. These polyisocyanate compounds can be used individually andcombinations of two or more types can also be used.

Examples of the blocked polyisocyanate compounds include those where theisocyanate groups of a polyisocyanate compound have been blocked with,for example, an alcohol such as butanol or the like, an oxime such asmethyl ethyl ketone oxime or the like, a lactam such as ε-caprolactam orthe like, a diketone such as an acetoacetic acid diester or the like, animidazole such as imidazole, 2-ethylimidazole or the like or a phenolsuch as m-cresol or the like.

The solid fraction mass content proportion with respect to the paintfilm-forming resin (A) of the crosslinking agent (C) in a chromium-freepaint composition of this invention is, from the viewpoints of corrosionresistance and workability, preferably from 3 to 60 mass %, moredesirably from 5 to 50 mass %, and most desirably from 10 to 40 mass %.

In addition to the components indicated above, the various knowncomponents which are generally used in the paint field can be included,as required, in a chromium-free paint composition of this invention. Inmore practical terms examples include various surface controlling agentssuch as leveling agents, anti-foaming agents and the like, variousadditives such dispersing agents, settling inhibitors, ultravioletabsorbers, light stabilizers and the like, various pigments such ascoloring pigments, true pigments and the like, glitter materials, curingcatalysts, organic solvents and the like.

A paint composition of this invention may be an organic solvent typepaint or an aqueous paint, but it is preferably an organic solvent typepaint. The organic solvents include one type, or a mixture of two ormore types, of, for example, the ketone-based solvents such ascyclohexanone and the like, the aromatic solvents such as Solvesso 100(trade name, produced by the Exxon Mobil Chemical Co.) and the like, andthe alcohol-based solvents such as butanol and the like.

Examples of the objects which are to be coated with a chromium-freepaint composition of this invention are those where treatment with achromium-free or chromate-based chemical forming agent or the like hasbeen carried out on various types of zinc-plated steel sheet such asmolten-zinc-plated steel sheet, electro-zinc-plated steel sheet,alloying zinc-plated steel sheet, aluminum-zinc-plated steel sheet,nickel-zinc-plated steel sheet, magnesium-aluminum-zinc-plated steelsheet, magnesium-aluminum-silica-zinc-plated steel sheet and the like,stainless steel sheet, aluminum sheet and the like. The use of achromium-free chemical forming treatment agent at the time of thechemical forming treatment is preferred.

Generally a top-coat paint is painted over the undercoat paint in theproduction of pre-coated steel sheets. It is possible to provide apre-coated steel sheet with a beautiful appearance by coating a top-coatpaint and, furthermore, it is possible to raise various aspects ofperformance such as the workability, weather resistance, resistance tochemical attack, staining resistance, water resistance, corrosionresistance and the like which are required of a pre-coated steel sheet.

No particular limitation is imposed upon the application in which achromium-free paint composition of this invention is used, but it ispreferably used as an undercoat paint in the production of pre-coatedsteel sheets.

Polyester resin-based paints, silicon-polyester resin-based paints,polyurethane resin-based paints, acrylic resin-based paints, fluorinatedresin-based paints and the like can be cited as examples of the top-coatpaints when a chromium-free paint composition of this invention is usedas an undercoat paint in the production of a pre-coated steel sheet.

The methods which are generally used in the production of pre-coatedsteel sheets, for example roll-coater coating, curtain-flow coatercoating and the like, can be adopted as the method of coating achromium-free paint composition of this invention.

The general coating conditions for the production of pre-coated steelsheets can be adopted as the coating conditions for a chromium-freepaint composition of this invention.

The coated film thickness of the undercoat paint in the production ofpre-coated steel sheets is, for example, from 1 to 30 μm, and theheating and curing conditions of the undercoat paint film are, forexample, a maximum sheet temperature reached of from 150 to 300° C. anda curing time of from 15 to 150 seconds.

The coated film thickness of the top-coat paint in the production ofpre-coated steel sheets is, for example, from 10 to 25 μm, and theheating and curing conditions of the top-coat paint film are, forexample, a maximum sheet temperature reached of from 190 to 250° C. anda curing time of from 20 to 180 seconds.

Moreover, one or more mid-coat paint films may be formed between theundercoat paint film and the top-coat paint film in accordance with theperformance required of the pre-coated steel sheet.

A chromium-free paint composition of this invention can also be used asa top-coat paint for coating the reverse side of the base steel sheet inthe production of pre-coated steel sheets. It is possible to obtaincoated metal sheets which have excellent corrosion resistance and whichare environmentally friendly, containing no chromium-based anti-rustpigment, by forming paint films by coating a chromium-free paintcomposition of this invention on both sides of the base steel sheet.

ILLUSTRATIVE EXAMPLES

The invention is described in more detail below by means of illustrativeexamples, but the invention is not limited by these examples. Moreover,in the absence of any indication to the contrary the terms “parts”, “%”and “ratio” in the examples signify “parts by mass”, “mass %” and “massratio” respectively.

Example of Production 1-1: Preparation of Anti-Rust Pigment B1

Magnesium oxide (470 g) and 530 g of vanadium pentoxide were added to 10L of deionized water and, after raising the temperature to 60° C., themixture was stirred for 2 hours at the same temperature. The reactionproduct obtained was de-watered after being washed with water and thendried at 100° C. and pulverized to provide the amorphous MgO—V₂O₅-basedcompound 1. This was the anti-rust pigment B1.

Example of Production 1-2: Preparation of Anti-rust Pigment B2

The amounts of the raw materials used were changed to 530 g of magnesiumoxide and 470 g of vanadium pentoxide and amorphous MgO—V₂O₅-basedcompound 2 was obtained with the same method as in Example of Production1-1. This was the anti-rust pigment B2.

Example of Production 1-3: Preparation of Anti-rust Pigment B3

The amounts of the raw materials used were changed to 570 g of magnesiumoxide and 430 g of vanadium pentoxide and amorphous MgO—V₂O₅-basedcompound 3 was obtained with the same method as in Example of Production1-1. This was the anti-rust pigment B3.

Example of Production 1-4: Preparation of Anti-Rust Pigment B4

The amounts of the raw materials used were changed to 640 g of magnesiumoxide and 360 g of vanadium pentoxide and amorphous MgO—V₂O₅-basedcompound 4 was obtained with the same method as in Example of Production1-1. This was the anti-rust pigment B4.

Example of Production 1-5: Preparation of Anti-rust Pigment B5

The amounts of the raw materials used were changed to 670 g of magnesiumoxide and 330 g of vanadium pentoxide and amorphous MgO—V₂O₅-basedcompound 5 was obtained with the same method as in Example of Production1-1. This was the anti-rust pigment B5.

Example of Production 1-6: Preparation of Anti-rust Pigment B6

The amounts of the raw materials used were changed to 400 g of magnesiumoxide and 600 g of vanadium pentoxide and amorphous MgO—V₂O₅-basedcompound 6 was obtained with the same method as in Example of Production1-1. This was the anti-rust pigment B6.

Example of Production 1-7: Preparation of Anti-Rust Pigment B7

The amorphous MgO—V₂O₅-based compound 5 (500 g) and 500 g of theamorphous MgO—V₂O₅-based compound 6 were uniformly mixed using a pestleand mortar and the anti-rust pigment B7 was obtained.

Example of Production 1-8: Preparation of Anti-Rust Pigment B8

The amounts of the raw materials used were changed to 730 g of magnesiumoxide and 270 g of vanadium pentoxide and amorphous MgO—V₂O₅-basedcompound 8 was obtained with the same method as in Example of Production1-1. This was the anti-rust pigment B8.

Example of Production 1-9: Preparation of Anti-rust Pigment 89

The amounts of the raw materials used were changed to 250 g of magnesiumoxide and 750 g of vanadium pentoxide and amorphous MgO—V₂O₅-basedcompound 9 was obtained with the same method as in Example of Production1-1. This was the anti-rust pigment B9.

Example of Production 1-10: Preparation of Anti-rust Pigment B10

The amounts of the raw materials used were changed to 740 g of magnesiumoxide and 260 g of vanadium pentoxide and amorphous MgO—V₂O₅-basedcompound 10 was obtained with the same method as in Example ofProduction 1-1. This was the anti-rust pigment B10.

Example of Production 1-11: Preparation of Anti-rust Pigment B11

The amorphous MgO—V₂O₅-based compound 8 (600 g) and 400 g of theamorphous MgO—V₂O₅-based compound 10 were uniformly mixed using a pestleand mortar and the anti-rust pigment B11 was obtained.

Moreover, the grain sizes of the anti-rust pigments B1 to B11 obtainedin the examples of production outlined above were within the range from10 to 30 μm.

A 10 mass % aqueous solution of the anti-rust pigment was prepared witheach of the anti-rust pigments B1 to B11 and the total eluted ioncontents and pH values were measured with the methods indicated below.

Preparation of a 10 Mass % Aqueous Solution of an Anti-Rust Pigment

The anti-rust pigment (10 g) and 90 g of ion-exchanged water were addedto a wide-necked bottle made of polyethylene. The lid was attached andthe liquid suspension obtained on shaking vigorously for 1 minute wasleft to stand for 24 hours at room temperature and a 10 mass % aqueoussolution of the anti-rust pigment was obtained.

Measurement of the Total Eluted Ion Content and pH

The supernatant liquid was collected from each of the mass % aqueoussolutions of anti-rust pigment obtained with the method outlined aboveand, using the filtrate obtained by filtration through a rayon filterpaper as the sample, the total eluted ion content was measured using ICPemission spectral analysis apparatus (JY-328 Ultrace, produced by theHoriba Seisakujo Co.). Furthermore, the pH was measured with a pH meter(HM-20S, produced by the Toa Denpa Kogyo Co.). The measured results aresummarized in Table 1.

TABLE 1 Total Eluted Ion Content pH of a of a 10 mass % 10 mass %Anti-rust Mg/V Aqueous Solution Aqueous Pigment (B) (mol ratio) (ppm)Solution B1  2.0 43.2 10.7 B2  2.5 39.6 10.8 B3  3.0 33.7 10.8 B4  4.020.1 10.9 B5  4.6 13.4 10.8 B6  1.5 70.6 10.5 B7  3.0 27.8 10.8 B8  6.16.2 11.8 B9  0.8 144 10.1 B10 3.1 31.9 8.8 B11 4.9 12.3 11.2

Example 1: Production of Chromium-Free Paint Composition CF1

Epoxy resin (trade name jER 1009, bisphenol A-type epoxy resin, producedby the Mitsubishi Kagaku Co., 80 parts) was heated and dissolved in 120parts of a mixed solvent (aromatic solvent (trade name Solvesso 100,produced by the Exxon Mobil ChemicalCo.)/cyclohexanone/n-butanol=55/27/18 (mass ratio)) using a flask whichhad been furnished with stirring apparatus, a condenser and athermometer and an epoxy resin solution which formed a paintfilm-forming component (A) was obtained. Next 30 parts of cyclohexanoneand 30 parts of aromatic solvent (trade name Solvesso 150, produced bythe Exxon Mobil Chemical Co.) were added to 200 parts of this epoxyresin solution, 15 parts of the anti-rust pigment B1 were added anddispersed in a sand grind mill until the grain sized reached from 20 to25 μm and a mill base was obtained. A blocked isocyanate compound (tradename Desmodur BL-3175, produced by the Sumika Beyer Urethane Co, 21parts) and 0.3 part of dibutyltin dilaurate (DBTDL) were added to thismill base and mixed uniformly and the chromium-free paint compositionCF1 was obtained.

Examples 2 to 34 and Comparative Examples 1 to 11: Production ofChromium-Free Paint Compositions CF2 to 45

The chromium-free paint compositions CF2 to 45 were obtained with thesame method as in Example 1 in accordance with the compoundingcompositions shown in Tables 2 to 8.

TABLE 2 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Chrome-free PaintComposition CF1 CF2 CF3 CF4 CF5 CF6 CF7 (A) Paint Epoxy resin solution200  200  200  200  200  200  200  Film- (Note 1) Forming Modified EpoxyResin Resin (Note 2) Polyester Resin (Note 3) (C) Blocked polyisocyanate21 21 21 21 21 21 21 Cross- Compound (Note 4) linking Melamine ResinAgent (Note 5) (B) B1 45 Anti-rust B2 45 Pigment B3 15 45 70 B4 45 B5 45B6 B7 B8 B9 B10 B11 Pigment Titanium Dioxide(Note 9) Kaolin (Note 10)Precipitated Barium Sulfate (Note 11) Finely Powdered Silica (Note 12)Curing DBTDL (Note 13)   0.3   0.3   0.3   0.3   0.3   0.3   0.3Catalyst Organic Aromatic Solvent 30 30 30 30 30 30 30 Solvent (Note 14)Cyclohexanone 30 30 30 30 30 30 30 (B) Component/{(A) Component + (C)16% 47% 73% 47% 47% 47% 47% Component} (mass %) (C) Component/(A)Component (mass %) 20% 20% 20% 20% 20% 20% 20% Paint Film Adhesion onBase Steel Sheet ◯ ◯ ◯ ◯ ◯ ◯ ◯ Performance Boiling Water Resistance

Evaluation (Paint film anomalies) Boiling Water Resistance

(Adhesion on Base Steel Sheet) Bending Workability (2T)

Bending Workability (3T)

Corrosion Resistance ◯

◯

(4T Bend Worked Part) Corrosion Resistance ◯

(Cross-cut Part) Corrosion Resistance ◯

◯ (Edge Part)

TABLE 3 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex. 12 Ex. 13 Ex. 14 Chrome-free PaintComposition CF8 CF9 CF10 CF11 CF12 CF13 CF14 (A) Paint Epoxy resinsolution 200  200  200  200  200  200  200  Film- (Note 1) FormingModified Epoxy Resin Resin (Note 2) Polyester Resin (Note 3) (C) Blockedpolyisocyanate 21 21 21 21 21 21 16 Cross- Compound (Note 4) linkingMelamine Resin  5 Agent (Note 5) (B) B1 Anti-rust B2 Pigment B3 45 45 45B4 B5 B6 45 B7 45 B8 B9 B10 45 B11 45 Pigment Titanium Dioxide(Note 9)15 15 Kaolin (Note 10)  5  5 Precipitated Barium Sulfate  5 (Note 11)Finely Powdered Silica  5 (Note 12) Curing DBTDL   0.3   0.3   0.3   0.3  0.3   0.3   0.3 Catalyst (Note 13) Organic Aromatic Solvent 30 30 3030 30 30 30 Solvent (Note 14) Cyclohexanone 30 30 30 30 30 30 30 (B)Component/{(A) Component + (C) 47% 47% 47% 47% 47% 47% 47% Component}(mass %) (C) Component/(A) Component (mass %) 20% 20% 20% 20% 20% 20%19% Paint Film Adhesion on Base Steel Sheet ◯ ◯ ◯ ◯ ◯ ◯ ◯ PerformanceBoiling Water Resistance

Evaluation (Paint film anomalies) Boiling Water Resistance

(Adhesion on Base Steel Sheet) Bending Workability (2T)

Bending Workability (3T)

Corrosion Resistance ◯

◯ ◯

(4T Bend Worked Part) Corrosion Resistance

(Cross-cut Part) Corrosion Resistance ◯

◯ ◯

(Edge Part)

TABLE 4 Ex. 15 Ex. 16 Ex. 17 Ex. 18 Ex. 19 Ex. 20 Ex. 21 Chrome-freePaint Composition CF15 CF16 CF17 CF18 CF19 CF20 CF21 (A) Paint Epoxyresin solution 200  200  Film- (Note 1) Forming Modified Epoxy Resin200  Resin (Note 2) Polyester Resin 145  145  145  145  (Note 3) (C)Blocked polyisocyanate 50 60 21 21 21 21 Cross- Compound (Note 4)linking Melamine Resin 10 Agent (Note 5) (B) B1 45 Anti-rust B2 PigmentB3 45 45 45 15 45 70 B4 B5 B6 B7 B8 B9 B10 B11 Pigment TitaniumDioxide(Note 9) Kaolin (Note 10) Precipitated Barium Sulfate (Note 11)Finely Powdered Silica (Note 12) Curing DBTDL (Note 13)  0   0.6   0.3  0.3   0.3   0.3   0.3 Catalyst Organic Aromatic Solvent 30 30 30 30 3030 30 Solvent (Note 14) Cyclohexanone 30 30 30 30 30 30 30 (B)Component/{(A) Component + (C) 52% 38% 36% 16% 47% 73% 47% Component}(mass %) (C) Component/(A) Component (mass %)  8% 47% 56% 20% 20% 20%20% Paint Film Adhesion on Base Steel Sheet ◯ ◯ ◯ ◯ ◯ ◯ ◯ PerformanceBoiling Water Resistance

Evaluation (Paint film anomalies) Boiling Water Resistance

(Adhesion on Base Steel Sheet) Bending Workability (2T) ◯ ◯ ◯

Bending Workability (3T)

◯

Corrosion Resistance

◯ ◯

◯ (4T Bend Worked Part) Corrosion Resistance

◯

(Cross-cut Part) Corrosion Resistance

◯

(Edge Part)

TABLE 5 Ex. 22 Ex. 23 Ex. 24 Ex. 25 Ex. 26 Ex. 27 Ex. 28 Chrome-freePaint Composition CF22 CF23 CF24 CF25 CF26 CF27 CF28 (A) Paint Epoxyresin solution Film- (Note 1) Forming Modified Epoxy Resin Resin (Note2) Polyester Resin 145  145  145  145  145  145  145  (Note 3) (C)Blocked polyisocyanate 21 21 21 21 21 21 21 Cross- Compound (Note 4)linking Melamine Resin Agent (Note 5) (B) B1 Anti-rust B2 45 Pigment B3B4 45 B5 45 B6 45 B7 45 B8 B9 B10 45 B11 45 Pigment TitaniumDioxide(Note 9) Kaolin (Note 10) Precipitated Barium Sulfate (Note 11)Finely Powdered Silica (Note 12) Curing DBTDL (Note 13)   0.3   0.3  0.3   0.3   0.3   0.3   0.3 Catalyst Organic Aromatic Solvent 30 30 3030 30 30 30 Solvent (Note 14) Cyclohexanone 30 30 30 30 30 30 30 (B)Component/{(A) Component + (C) 47% 47% 47% 47% 47% 47% 47% Component}(mass %) (C) Component/(A) Component (mass %) 20% 20% 20% 20% 20% 20%20% Paint Film Adhesion on Base Steel Sheet ◯ ◯ ◯ ◯ ◯ ◯ ◯ PerformanceBoiling Water Resistance

Evaluation (Paint film anomalies) Boiling Water Resistance

(Adhesion on Base Steel Sheet) Bending Workability (2T)

Bending Workability (3T)

Corrosion Resistance

◯

◯ ◯ (4T Bend Worked Part) Corrosion Resistance

(Cross-cut Part) Corrosion Resistance

◯ ◯

◯ ◯ (Edge Part)

TABLE 6 Ex. 29 Ex. 30 Ex. 31 Ex. 32 Ex. 33 Ex. 34 Chrome-free PaintComposition CF29 CF30 CF31 CF32 CF33 CF34 (A) Paint Epoxy resin solutionFilm- (Note 1) Forming Modified Epoxy Resin Resin (Note 2) PolyesterResin 145  145  145  145  145  145  (Note 3) (C) Blocked polyisocyanate21 21 16 50 60 Cross- Compound (Note 4) linking Melamine Resin  5 10Agent (Note 5) (B) B1 Anti-rust B2 Pigment B3 45 45 45 45 45 45 B4 B5 B6B7 B8 B9 B10 B11 Pigment Titanium Dioxide(Note 9) 15 15 Kaolin (Note 10) 5  5 Precipitated Barium Sulfate  5 (Note 11) Finely Powdered Silica  5(Note 12) Curing DBTDL (Note 13)   0.3   0.3   0.3  0   0.6   0.6Catalyst Organic Aromatic Solvent 30 30 30 30 30 30 Solvent (Note 14)Cyclohexanone 30 30 30 30 30 30 (B) Component/{(A) Component + (C) 47%47% 47% 52% 38% 36% Component} (mass %) (C) Component/(A) Component(mass %) 20% 20% 19%  8% 47% 56% Paint Film Adhesion on Base Steel Sheet◯ ◯ ◯ ◯ ◯ ◯ Performance Boiling Water Resistance

Evaluation (Paint film anomalies) Boiling Water Resistance

(Adhesion on Base Steel Sheet) Bending Workability (2T)

◯ ◯ ◯ Bending Workability (3T)

◯ Corrosion Resistance

◯ (4T Bend Worked Part) Corrosion Resistance

(Cross-cut Part) Corrosion Resistance

(Edge Part)

TABLE 7 Comp. Comp. Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex.5 Ex. 6 Chrome-free Paint Composition CF35 CF36 CF37 CF38 CF39 CF40 (A)Paint Epoxy resin solution 200  200  200  200  200  200  Film- (Note 1)Forming Modified Epoxy Resin Resin (Note 2) Polyester Resin (Note 3) (C)Blocked polyisocyanate 21 21 21 21 21 21 Cross- Compound (Note 4)linking Melamine Resin Agent (Note 5) (B) B1 Anti-rust B2 Pigment B3  8B4 B5 B6 B7 B8 45 B9 45 B10 B11 Other Anti- Vanadium Pentoxide (Note 6)45 20 rust Magnesium Oxide (Note 7) 25 Pigments Calcium Vanadate (Note8) 45 Pigment Titanium Dioxide(Note 9) Kaolin (Note 10) PrecipitatedBarium Sulfate (Note 11) Finely Powdered Silica (Note 12) Curing DBTDL(Note 13)   0.3   0.3   0.3   0.3   0.3   0.3 Catalyst Organic AromaticSolvent 30 30 30 30 30 30 Solvent (Note 14) Cyclohexanone 30 30 30 30 3030 (B) Component/{(A) Component + (C)  0%  0%  0% 47% 47%  8% Component}(mass %) (C) Component/(A) Component (mass %) 20% 20% 20% 20% 20% 20%Paint Film Adhesion on Base Steel Sheet ◯ ◯ ◯ ◯ ◯ ◯ Performance BoilingWater Resistance

X

◯

Evaluation (Paint film anomalies) Boiling Water Resistance

X

X

(Adhesion on Base Steel Sheet) Bending Workability (2T)

◯ ◯ ◯ ◯

Bending Workability (3T)

◯ ◯ ◯

Corrosion Resistance X X X ◯ X

(4T Bend Worked Part) Corrosion Resistance ◯ X ◯ ◯ X ◯ (Cross-cut Part)Corrosion Resistance X X X X X X (Edge Part)

TABLE 8 Comp. Comp. Comp. Comp. Comp. Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11Chrome-free Paint Composition CF41 CF42 CF43 CF44 CF45 (A) Paint Epoxyresin solution 200  Film- (Note 1) Forming Modified Epoxy Resin Resin(Note 2) Polyester Resin 145  145  145  145  (Note 3) (C) Blockedpolyisocyanate 21 21 21 21 21 Cross- Compound (Note 4) linking MelamineResin Agent (Note 5) (B) B1 Anti-rust B2 Pigment B3 90  8 90 B4 B5 B6 B7B8 45 B9 45 B10 B11 Other Anti- Vanadium Pentoxide (Note 6) rustMagnesium Oxide (Note 7) Pigments Calcium Vanadate (Note 8) PigmentTitanium Dioxide(Note 9) Kaolin (Note 10) Precipitated Barium Sulfate(Note 11) Finely Powdered Silica (Note 12) Curing DBTDL (Note 13)   0.3  0.3   0.3   0.6   0.3 Catalyst Organic Aromatic Solvent 30 30 30 30 30Solvent (Note 14) Cyclohexanone 30 30 30 30 30 (B) Component/{(A)Component + (C) 94% 47% 47%  8% 94% Component} (mass %) (C)Component/(A) Component (mass %) 20% 27% 27% 27% 27% Paint Film Adhesionon Base Steel Sheet X ◯ ◯ ◯ X Performance Boiling Water Resistance ◯

◯

◯ Evaluation (Paint film anomalies) Boiling Water Resistance X

X

X (Adhesion on Base Steel Sheet) Bending Workability (2T) ◯ ◯ ◯

◯ Bending Workability (3T) ◯

◯

◯ Corrosion Resistance X ◯ X

X (4T Bend Worked Part) Corrosion Resistance

◯ X ◯

(Cross-cut Part) Corrosion Resistance X X X X X (Edge Part)

Details of each of the compounded components shown in Tables 2 to 8 areindicated below.

(Note 1) The epoxy resin solution was obtained by heating and dissolving80 parts of epoxy resin (trade name jER 1009, produced by the MitsubishiKagaku Co., bisphenol A-type epoxy resin, resin solid fraction 100 mass%, epoxy equivalent 2,500 to 3,500, number average molecular weight3,800) in 120 parts of mixed solvent (aromatic solvent (trade nameSolvesso 100, produced by the Exxon Mobil ChemicalsCo.)/cyclohexanone/n-butanol=55/27/18).(Note 2) Modified epoxy resin (trade name Epiclon H-304-40, produced bythe DIC Co., diethanolamine modified epoxy resin, resin solid fraction40 mass %, number average molecular weight 3,500)(Note 3) Polyester resin (trade name LH-822, produced by the EbonikDegussa Co., resin solid fraction 55 mass %, number average molecularweight 5,000, hydroxyl group value 50 mgKOH/g)(Note 4) Blocked polyisocyanate compound (trade name Desmodur BL-3175,produced by the Sumika Beyer Urethane Co., HDI isocyanurate typepolyisocyanate compound blocked with methyl ethyl ketone oxime, resinsolid fraction 75 mass %, NCO approximately 11.1 mass %)(Note 5) n-Butylated melamine resin (trade name Yuban 122, produced bythe Mitsui Kagaku Co., resin solid fraction 60 mass %)(Note 6) Vanadium pentoxide: commercial reagent(Note 7) Magnesium oxide: commercial reagent(Note 8) Calcium vanadate: Commercial reagent(Note 9) Titanium dioxide (trade name R-960, produced by the DuPont Co.)(Note 10) Kaolin (trade name Hydrite MS, produced by Imerys MineralsCo.)(Note 11) Precipitated barium sulfate (trade name SS-50, produced by theSakai Kagaku Kogyo Co.)(Note 12) Finely powdered silica (trade name Nipsil E-200A, produced bythe Toso Silica Co.)(Note 13) DBTDL (produced by the Nitto Kase Co., involatile fraction100%)(Note 14) Aromatic solvent (trade name Solvesso 150, produced by theExxon Mobil Chemical Co.)

Test pieces on which a chromium-free paint composition of this inventionhad been used as an undercoat paint were prepared with the methodoutlined below with each of the chromium-free paint compositions CF1 toCF45 and performance evaluations of the paint films were carried out.

Preparation of a Test Piece

A chromium-free paint composition of this invention was coated with abar coater in such a way as to provide a dry film thickness of 5 μm onan aluminum/zinc alloy plated steel sheet of thickness 0.35 mm which hadbeen subjected to a chemical forming treatment and baked in a hot-draftdrier for 40 seconds with a maximum temperature reached by the sheet of210° C. and a undercoat paint film was formed. A polyester resin-basedtop-coat paint (trade name Precolor HD0030, produced by the BASFCoatings Japan Co. Ltd., brown color) was coated over the undercoatpaint film with a bar coater in such a way as to provide a dry filmthickness of 15 μm and baked in a hot-draft drier for 40 seconds with amaximum temperature reached by the sheet of 220° C., a top-coat paintfilm was formed and a test piece was obtained.

The test piece obtained was subjected to the paint film performanceevaluations indicated below and the results have been shown in Tables 2to 8.

Adhesion on the Base Steel Sheet

A square pattern of one hundred 1 mm×1 mm squares was formed in thepaint film of a test piece using a cutter knife. The part of the paintfilm on which the pattern had been formed was pushed out from thereverse side of the test piece with a punch in such a way that thedistance from the pushed-out surface of the test piece to the tip of thepunch was 6 mm using an Erichsen testing machine. Subsequentlycellophane tape was pressed and stuck firmly over the pattern part ofthe pushed-out paint film and peeled off in one action with the end ofthe tape at an angle of 45°, the state of the pattern was observed andan evaluation was made on the basis of the following criteria:

◯: No peeling of the paint film was observed.

X: Peeling of the paint film was observed.

Boiling Water Resistance

A test piece was immersed for 2 hours in boiling water and allowed tocool for 2 hours at room temperature in accordance with JIS K 5600-6-2and then the test piece was evaluated using the methods (1) and (2)below.

(1) Paint Film Abnormality

The paint film of the test piece was observed for abnormalities andevaluated on the basis of the following criteria.

: No paint film abnormality at all was observed.

◯: Slight bulging of the paint film was observed.

X: Distinct bulging of the paint film was observed.

(2) Adhesion on the Base Metal Sheet

A square pattern of one hundred 1 mm×1 mm squares was formed with acutter knife in the paint film of a test piece, cellophane tape wasfirmly stuck over the pattern part and peeled away with the end of thetape at an angle of 45° in accordance with JIS-K 5600-5-6, the state ofthe pattern was observed and an evaluation was made in accordance with“Table 1, Classification of Test Results” of JIS K 5600-5-6.

: Class 0

◯: Class 1

X Classes 2 to 5

Bending Workability

A test piece was bent through 180° in such a way that sheets similar tothe test piece were inserted. At this time the number of sheets similarto the test piece within the test piece is indicated by 0T, 2T and thelike. For example, 0T indicates that the test piece was bent with nosheet similar to the test piece inserted and 2T indicates that the testpiece was bent in such a way that two sheets similar to the test piecewere inserted. In the performance evaluation of this invention 2T and 3Ttests were carried out and cellophane tape was firmly stuck over theapex part after bending, the tape was peeled away in one action with theend of the tape at an angle of 45° and an assessment was made on thebasis of the following criteria depending on the state of peeling of thepaint film.

: No peeling of the paint film was observed.

◯: Slight peeling of the paint film was observed.

X: Peeling of the paint film was observed.

Corrosion Resistance

Corrosion resistance test pieces were prepared in accordance with (1) to(3) below.

(1) A test piece of 70 mm×150 mm was cut in such a way that the edgepart of the test piece had a burr facing the side on which the paintfilm had been formed (up-burr) and a burr facing the opposite side tothat on which the paint film had been formed (down-burr).(2) The test piece was subjected to 4T bending.(3) A cross-cut of a depth which reached the base material surface wasmade in the middle part of the paint film on the side on which the paintfilm had been formed in such a way that it did not extend over the 4Tbend part of (2).

The prepared corrosion resistance test piece was subjected to a saltwater spray test (SST) of duration 500 hours in accordance with JISK5600-7-1 and the state of the edge part, the 4T bend part and the crosscut part of the corrosion resistance test piece after the test wereevaluated on the basis of the following criteria:

Edge Part

The widths of the edge creep of the up-burr and the down-burr weremeasured and evaluated on the basis of the following criteria:

: The average value of the edge creep width of the up-burr and thedown-burr was less than 4 mm.

◯: The average value of the edge creep width of the up-burr and thedown-burr was at least 4 mm but less than 10 mm.

X The average value of the edge creep width of the up-burr and thedown-burr was 10 mm or more.

4T Bend Part

The state of occurrence of white rust at the apex where the bending hadbeen carried out was evaluated on the basis of the following criteria:

: Virtually no white rust was observed.

◯: A little white rust was observed.

X: White rust was observed.

Cross-Cut Part

The state of occurrence of white rusting of the cross cut part wasobserved and the width of the swollen paint film was measured andevaluated on the basis of the following criteria:

: Virtually no white rust was observed and the width of the bulgingpaint film was less than 2 mm.

◯: Some white rust was observed and the width of the bulging paint filmwas at least 2 mm but less than 5 mm.

X: White rust was observed and the width of the bulging paint film was 5mm or more.

The invention claimed is:
 1. A chromium-free paint compositioncomprising a paint film forming resin (A), an anti-rust pigment (B) thatcomprises at least one amorphous MgO—V₂O₅ compound, and a crosslinkingagent (C), wherein the mass content proportion of the anti-rust pigment(B) is in the range of from 20 to 80 mass % with respect to the sum ofthe resin solid fraction mass of the paint film-forming resin (A) andthe crosslinking agent (C), the mol ratio of magnesium with respect tovanadium is in the range of from 1.7 to 5.0, and wherein the overalleluted ion content in a 10% aqueous solution of the anti-rust pigment(B) is from 10 ppm to 100 ppm and the pH of the 10% aqueous solution isin the range of 9.0 to 11.0, and wherein the amorphous MgO—V₂O₅ isproduced by mixing and reacting magnesium-containing compounds andvanadium-containing compounds in water, drying, and pulverization. 2.The chromium-free paint composition according to claim 1, wherein themagnesium compound which is a raw material for production of theamorphous MgO—V₂O₅ compound is at least one magnesium-containingcompound selected from the group consisting of magnesium oxide andmagnesium carbonate.
 3. The chromium-free paint composition according toclaim 1, wherein the vanadium-containing compound which is a rawmaterial for production of the amorphous MgO—V₂O₅ compound is vanadiumpentoxide.
 4. The chromium-free paint composition according to claim 1,wherein the paint film-forming resin (A) is at least one paintfilm-forming resin selected from the group consisting of hydroxylgroup-containing epoxy resins of number average molecular weight from400 to 10,000 and hydroxyl-group containing polyester resins of numberaverage molecular weight from 500 to 20,000.
 5. The chromium-free paintcomposition according to claim 1, wherein the crosslinking agent (C) isat least one crosslinking agent selected from the group consisting ofblocked polyisocyanate compounds and amino resins, and wherein the solidfraction mass content proportion of the crosslinking agent (C) is from 3to 60 mass % with respect to the solid fraction mass of the paintfilm-forming resin (A).
 6. A paint film that has been obtained bycoating the chromium-free paint composition according to
 1. 7. Apre-coated steel sheet comprising a steel sheet and the chromium-freepaint composition according to claim 1 coated thereon.
 8. Achromium-free paint composition comprising a paint film forming resin(A), an anti-rust pigment (B) that comprises at least one amorphousMgO—V₂O₅ compound, and a crosslinking agent (C), wherein the masscontent proportion of the anti-rust pigment (B) is in the range of from10 to 80 mass % with respect to the sum of the resin solid fraction massof the paint film-forming resin (A) and the crosslinking agent (C), andwherein the overall eluted ion content in a 10% aqueous solution of theanti-rust pigment (B) is from 10 ppm to 100 ppm, and wherein anti-rustpigment (B) is used without conjoint use of other anti-rust pigments,and wherein the amorphous MgO—V₂O₅ is produced by mixing and reactingmagnesium-containing compounds and vanadium-containing compounds inwater, drying, and pulverization.
 9. A chromium-free paint compositioncomprising a paint film forming resin (A), an anti-rust pigment (B) thatcomprises at least one amorphous MgO—V₂O₅ compound, and a crosslinkingagent (C), wherein the mass content proportion of the anti-rust pigment(B) is in the range of from 10 to 80 mass % with respect to the sum ofthe resin solid fraction mass of the paint film-forming resin (A) andthe crosslinking agent (C), and wherein the overall eluted ion contentin a 10% aqueous solution of the anti-rust pigment (B) is from 10 ppm to100 ppm, and wherein anti rust pigment (B) the amorphous MgO—V₂O₅ isproduced by mixing and reacting magnesium-containing compounds andvanadium-containing compounds in water, drying, and pulverization. 10.The chromium-free paint composition according to claim 9, wherein theanti-rust pigment (B) has a mol ratio of magnesium to vanadium of from1.7 to 5.0.